The origin and nature of brain waves as measured from the human scalp has been a topic of ongoing research. For example, the electroencephalogram (EEG) as measured from the intact human scalp is of interest in psychology and psychophysics because it can provide an indication of the activity of brain cells in the awake, alert state. Of particular use are the minute potentials evoked by sensory stimuli, for these time-locked transient wavelets show how populations of cells behave in response to afferent volleys carried by primary sensory fibers.
Whenever a brief stimulus is presented to a trainee, there is a transient brain response due to that stimulation. The signal produced in the EEG is generally very small, but it can be detected. In cases where it is possible to discern the EEG changes, either in the raw EEG or in a processed form, then there is said to be an event-related potential (ERP), particularly a sensory evoked potential. The evoked potential provides an indication of the effect of the stimulus on the brain, and it has been established that the EP is sensitive to changes in sensory and perceptual processes.
When muscle and eye movements are minimized by relaxation, the predominant sources of scalp potential are these populations of cells in the brain, with large cortical cells providing the majority of the voltage. Due to the distribution of ions across active membranes, each cell tends to take on dipole characteristics and produce potentials which are carried to the scalp by volume conduction. When cells polarize in asynchrony, the net surface potential is small due to the cancellation of out-of-phase components. The presence of a measurable surface potential thus depends on the fact that some cells are polarizing in synchrony, generally in response to an afferent volley in which fibers are firing in unison.
The brain produces a multitude of frequencies that can be measured in the EEG. These can be broken into commonly recognized bands, including alpha (8-12), beta (12-20), and theta (4-7). It is found that the relative preponderance of these rhythms is a valuable indicator of global brain state, as well as short-term variations in brain state. In certain cases, it is possible to identify desirable as well as undesirable states, on the basis of the relative amounts of these rhythms. In particular, an excessive amount of theta waves is associated with a dreamlike, distracted, or inattentive state. In cases where it is desired to discourage these states, it is beneficial to have methods that encourage the brain to reduce the amount of these waves, on both a short-term basis, and in the long-term, as a method of training the brain to naturally reach more desirable states.
It has also been observed that sensory stimulation has specific effects on the EEG. These include both evoked potentials (signals that are introduced into the EEG due to the stimulation) and extinction phenomena (signals that are removed from the EEG due to the stimulation). In particular, the theta wave can be reduced by a stimulus that produces an alerting or re-orienting response. Such a stimulus has the effect of startling or alerting the brain, and can be presented in such a manner that endogenous rhythms (including theta and alpha) are extinguished, at least temporarily, by the presentation of the stimulation.
When such stimulation is provided in a contingent manner (depending on the details of the EEG at some time), then there is an opportunity for the brain to learn, either by classical conditioning, or by operant conditioning. It is thus desirable to develop methods that allow the selective presentation of specific stimuli, based upon parameters derived from the EEG. Such methods can provide nonvolitional feedback methods, that do not depend on the intentions or understanding of the trainee, thus providing robust and immediate mechanisms for brain modification.
At this time, the use of contingent stimulation in the context of EEG biofeedback (“neurofeedback”) is not a well developed field. In particular, there are no well-defined methods that use EEG parameters for the presentation of specifically activating (or de-activating) stimuli, in the context of brain modification via the EEG.